Gelation coating process



Patented June 6, 1944A or-'Fica' y 2.350.144 GELATION COATING PROCESS charles a. Forum mi Gema J. claire, nochester, N. Y., assignors to Eastman Kodak Oompany, Rochester, N. Y., a corporation of New Jersey Dividedamlthla tion December 19, 1940, Serial No.

application December 8, 1943, Serial No. 513,416

1 Claim. (Cl. 117-103) This invention relates to coating, and more particularly to a process for coating objects by dipping them in various cellulose ester lacquers which have the ability to solidify by gelation as distinguished from ordinary cellulose ester lacquers.

This application is a division of our copending application, Ser. No. 370,853, illed December 19, 1940.

As is well known, cellulose ester coatings may be applied to the surface of various objects made of wood, metal and other substances by dipping them in certain cellulose ester lacquer compositions, or by brushing or spraying such compositions onto the object. The dipping process involvesv the immersion of the article to be coated in the lacquer bath and gradual withdrawal therefrom with the object of. producing a uniformly even coating. The success of the operation depends upon proper control of uniformity of coating, both as to thickness and surface appearance. While the lacquer dipping process presents many desirable features for the economic application of surface coatings, especially on small articles, there are a number of outstanding dimculties which severely limit its use. For example, upon withdrawal of the object from the lacquer solution there is a pronounced tendency for the uncured coating to run,- building up ridges of uneven thickness upon drying. The coating of articles of irregular shape presents especial dimculty because of creeping of the fresh coating away from edges or corners. leaving a very thin coating at those places. In order to avoid these diiliculties, it is usually necessary to apply only a very 'thin coating to the object so that such' unevenness is less pronounced. However, under such circumstances, it is difilcult to buildup a heavier coverage by repe'ating'dipping because the solvents in the dipping bath attack the previously applied coating, causing it to become even more uneven and, in some cases, actually to re- .move it completely on various localized areas.

Various attempts have been made to overcome the above-mentioned diiliculties which are common to all conventinal cellulose ester lacquers. lIt has been proposed, for example, to dissolve cellulose acetate in a solvent or solvent mixture which is a solvent only atan elevated temperature, dip the article in such a solution and cool the object until solidication of the coating was effected. Solvent mixtures of alcohols and hydrocarbons have been suggested for this purpose. but such an expedient has never proved satisreason that such solutions undergo such a severe or sudden solidillcation upon cooling that the cellulose ester material precipitates from the solvent giving white or opaque coatings which, upon curing, are extremely brittle and thus entirely unsatisfactory. Such coatings have such a pronounced tendency toward separation of the liquid from the solid material as to give an effect which is known as watering, rather than undergoing homogeneous evaporation as is the case with customary lacquer solvents. A further attempt to eliminate'some of the above-mentioned difilculties encountered in the use of dipping lacquers is based upon the use of cellulose nitrate lacquers and the chilling of the dipper object in order to arrest fluidity of the coating to such a degree that running of the composition is less pronounced than with a lacquer employed at ordinary temperatures. This process has the disadvantages that such refrigeration is expensive and that only certain cellulose derivatives, speciilcally cellulose nitrates,.can be made to give satisfactory commercial operation.

This invention has as an object to provide an entirely new and improved method of producing cellulose ester coatings on the surface of various articles which can be immersed in a bath of-1 dipping lacquer. A further object is to provide a rapid and reliable method of producing uniform cellulose ester coatings onthe surface of such objects. A still further object is to provide a process of producing cellulose ester lacquer coatings which will not run, become uneven, or 'will not lift oif or develop thin spots upon the appli.- cation of successive lacquer layers. Another object is to provide a method of producing surface coatings on a wide variety of plain an'd irregularly surfaced objects not hitherto susceptible `of coating by ordinary lacquer technique. Other objects will appear hereinafter.

'I'hese objects are accomplished by the *following invention which, in its broader aspects, involves dipping the object to-be coated in asolution of a cellulose organic acid ester dissolved in a liquid which is a solvent for the ester in question only at temperatures above 50 C. and which at a temperature between l0 and 50 C. will form a self-supporting gel, said solution being maintained at a temperature above 50 C. removing the dipped article from the solution and thereafter cooling the dipped article to a temperature between 10 and 50 C. followed by curing of the residual solventtherefrom.

factory, or even commercially operative, for the y In the following examples and description, we

have set forth several of the preferred embodiments of our invention, but they are included merely for purposes of illustration and not as a limitation thereof.'

In the accompanying drawing we have Aillustrated one type of apparatus suitable for carrying out the process of our invention.

In this drawing:

Fig. V1 is a plan view of fone form of dipping apparatus suitable for the practice of our invenn. Y tlolllg. 2 is a sectional elevational view of the apparatus of Fig. l.

Referring to Figs. 1 and 2, numeral I designates a jacketed dipping tank for containing the bath of dipping lacquer 2. Any suitable heating fluid, such as hot water, steam or the like, may be supplied to jacket 3 of the tank through inlet conduit l, the fluid flow through which is controlled by valve 5. The heating fluid passes out Y Y ing any desired level of liquid therein. In the case of tank 9, the desired level may alternatively be maintained by an appropriate adjustment of valves and |2.

Positioned adjacent to both tanks is a lowering and elevating mechanism I4 comprising standard 'I5 which is bored to accommodate shaft I6 which carries head I1. Provision is made for raising or lowering the head on shaft I6 through collar I8 provided with adjustable screw member |9 threaded through the collar and adapted to bear against thevshaft thereby xing the position of head- "which rests thereon. Mechanism I4 is also provided with a windlass 20 the shaft 2| of which is rotatably mounted in suitable lbearings in lugs 22 integral with standard I5 adjacent its upper part. Shaft 2| is rotated by means of crank 23.

' A steel cable or cordzi is attached to the shaft vof the windlass as shown and passes over pulleys 25 and 26 rotatably mounted in the channel 2l as shown. The outer end of cord 24 is rove through eyelet 28 in dipping rack 29. This rack has' secured thereto a plurality of hooks 30 to which the objects 3| to be dipped are temporarily attached by any convenient means.

The operation of the apparatus will be apparent from a consideration of.Figs. 1 and 2. Dipping rack 29 is raised to an appropriate height by'winding' up cable 25 by means of windlass 20. 'I'he objects to be dipped are then attached to the hooks 39, head I1 is then swlmg over until rack 29 is directly over the dipping bath 2, whereupon the windlass is operated in the reverse direction, thus lowering the objects slowly into the body of dipping lacquer where they are allowed to stand until free from surface bubbles. The windlass is then operated slowly so as to evenly and gradually withdraw the objects from the lacquer at a uniform rate. time being allowed for the' objects to acquirey on their surface a layer of the lacquer of appropriate thickness.

'I'he tank, as shown, should preferably be so constructed as to protect the atmosphere above the surface of the'A dope from air currents and to provide a fairly high concentration of solvent vapors in that'vicinity. While no hard and fast rule can be laid down, it may be said that after V fingerprints or other contacts will lnot cause damage, notwithstanding that the coating retains practically all of the original volatile solvent at this stage of the operation. Following this gelation stage, rack 29 may then be wound up until all of the objects can clear the top of the tank. Head |1 is then swung on shaft I6 as a pivot andthe lacquer coated objects left to cure by evaporation of solvent, which may be carried'out at room temperature or in a moderately warm atmosphere.

As to the actual gelation step, it should be pointed out that if a dope is employed which gels at ordinary room temperatures, no positive cooling of the atmosphere in which gelation takes place is necessary. However, if a dope is used which gels only below room temperatures special provision must be made for refrigerating the at- `mosprhere in the immediate vicinity 0f the dipped objects. Alternatively, the objects may be dipped 30 in a bath of cold non-solvent liquid such as toluene. In the latter case the head l1 is swung over until the rack is in a position to be lowered into the bath contained in tank 9. This tank may, for example, contain a body of cold toluene maintained at any desired temperature by cooling coils or other appropriate device. Rack 29 is then lowered as in the previous dipping operation and the lacquered objects are immersed in the cold liquid for. an appropriate interval of time,

say for M; to 5 minutes.

The second step of dipping the lacquered objects in a' non-solvent liquid is also employed when itis desired .to deposit very heavy coatings, it having been found that when a.v heavy coating is deposited andl solidified by gelation as previously described, if the gelled coating is treated with a non-solvent liquid such as toluene, for example, the residual solvent may be removed from the coating much more evenly and uniformly than is the case where no non-solvent treatment is employed. 'I'his is presumably because of the extracting effect of the non-solvent liquid on the solvents contained in the gelled material, or the replacement of the original solvent in the gelled structure by a more volatile and therefore more easily removable solvent than 'the original liquid.

As to the non-solvent liquid itself, this may be any liquid which is non-solvent with respect to 00 the cellulose ester in question. However, we prefer to employ non-solvents such as toluene, carbon tetrachloride, xylene, benzene and ligroin. Of these liquids, toluene is probably the most satisfactory, for all-around operation. It may also be said that the non-solvent liquid is preferably to be selected upon the basis of its power of dissolving or extracting the solvent from the gelled composition, since this facilitates removal of solvent from the deposited coating and leaves less residual solvent to be removed in the air curing stage.

In the case of thick layers the non-solvent dipping treatment is especially desirable in that shrinkage of the deposited coating, which is considerable, takes placeuniformly throughout the costing layer. and unis precludes impossibility of buckling or distortion of lthe surface. This .y

i treatment may be unnecessary in the' case of veryithin coatings, but is desirable with heavier layers. In either case the deposited lacquer coating shrinks down tightly and evenly round the v object producing a highly satisfactory, .smooth The general nature of the solutions which may be employed in the lacquerln'g process of our irlvention have been .referred to above. These solutions are what may be called gel dopes or lac' quers, that is, cellulose ester solutions whichv are fluid above 50 C. but at a temperature` within the range of -50" C. set ,to amen-fluidselfsupporting gel. The characteristics of such compositions are more particularly set forth in the U. S. patents to Fordyce and Hunter 2,319,051, 2,319,052, 2,319,053, 2,319,054 and the U. S. patent to Fordyce and Gould 2,319,055. The instantapplication relates specifically to those compositions in which a gellable cellulose organic acid ester of the type referred to and more particularly illustrated by the specic examples to follow, is dissolved in a solvent composed of alkylene dichlorides of from 2 4 carbon atoms. such as ethylene, propylene or butylene chlorides. alone or in admixture. The cellulose esters whichhave been found most suitable are cellulose acetates of :iB-42% acetyl and cellulose acetate propionates or cellulose acetate butyrates of at least 43% total acyl content and containing not over 40% of the higher acyl radical.

The concentration of the lacquer solutions may vary depending upon the thickness of the coating desired, but will usually fall within the limits of 520% of the ester, based on the weight of the total composition. Plasticizers may be employed. if desired, up to an amount representing 50% of the cellulose ester. However, high plasticizer contents are to be avoided because of a tendency toward interference with the gelling properties of the composition. 0n the other-hand one of the outstanding advantages of the compositions of our invention is that one may depositv very heavy coatings Without the use o! any plasticizer whatever, as compared with molding operations usually employed to deposit such thick coatings which require considerable quantities of plasticizer in order to obtain a suitable ilpw under pressure.

Our invention will be more iully understood by -of ethylene dichloride at .C. with continuous stirring. The solution was filtered, then placed ina container suitable for dipping objects to be coated, such as the tank I of Figs. 1 and 2. and held at approximately 55 C. by means oi' jacket water at that temperature. The air immediately above the tank was maintained at 30-35" C. A

found to be .013" t ck. The heavy coating thus obtained gave the metal a brilliant appearance and protected it from corrosion 'and tarnish.

A steel spatula with a wooden handle was dipped in a similar way to apply a lacquer coat# ing to the handle.

obtained.

Example 2 lA solution of parts of'cellulose acetate butyrate of 16% butyryl and 32% acetyl content in 375 parts of propylene dichioride and parts of ethylene dichloride was prepared at 70 C.

'and there was added a pigment mixture of titanium oxide equal to 3% of the weight of the cellulose ester, to which had been added a small quantity of ultramarine blue dye toimprove color shade. The solution was well mixed, filtered, and placed in a dipping tank such as-that illustrated in Figs. 1 and 2 where it was maintained at 45 C. by jacket water.

A polished brass door mob was immersed in the solution and slowly withdrawn to give a uniform, heavy coating. After gelation and curing the coating was found tobe .013" in thickness and in appearance resembled a door knob of white plastic composition.

A number of glass bottles of irregular vshape were dipped. the coating gelled and cured as above described, resulting in coatings oi .011" thickness which were uniform in appearance.

A number of cardboard trays and plates were dipped in the lacquer, providing coatings which, when cured, were .006 thick, and which upon application of a second 'coat' were increased to .014". The coatings exhibited outstanding,'liial-l ities of coverage without penetrating deeply into the paper stock, a single coating being sumcient to completely y'surface the plates and make them resistant to warm or cold water.

Several pieces of pine wood. fiat surfaced on the bottom side and evenly rounded on the upper side and end, were prepared to represent wooden 'objects of uneven surface. They were sanded to produce a good finish and were then dipped. in the lacquer solution, the coating gelled as before. some being given a second coat after partial curing. The products were similar in appearance to white enameled coatings. The single coatings were found to be .011".thick and the double coatings .018" thick. The lacquer coatings gave very eicient surface coverage, penetrating only very slightly into the wood.

One of the outstanding features of our process is the fact that objects of a variety oi' shapes may be dipped and removed from -the dipping bath without rlmning of the lacquer, as is the case with prior art lacquersrwhichhave a pro nounced tendency to build up ridges of accumulated materialor to break away from sharp edges as previously described. Another feature of our invention is the fact that the solvents employed in these gel dope lacquers are'of such a nature thatsuccessive coatings maybe applied one after anhighly polished brass door knob vwas lowered intoA the solution, allowed to stand completely immersed for a few minutes, then withdrawn slowly. The freshly coated object could be handled without finger printing almost immediately upon its removal from the dipping solution. The coating was uniform in thickness andapncarance. It was cured by placing it in a 'oven at ,50 C.

' other without thinning or spotting out -oi' the lacquer previously deposited. This is primarily because of the fact that the superposed coating is applied at a temperature considerably below that at iwhich the solventsactively dissolve the cellulose ester and very little solvent action-is exerted upon the previously applied nlm. 1 Thus any numn ber of successiverlayerslmayrbe `deposited evenly and uniformly one after anthen The lacquer layers may co tsirifpigmenis or Y other coloring matter or maybe deposited clear,

` wooden object and, after appropriate solidiilcati'on by gelation and curing, the lacquered surface may be rubbed down with emery cloth, sand- !inishit desired.

.It goes without saying that almost any, form or shape of object may be lacquered by our imaiasonu occurs. Furthermore. since these coatings shrink paper, or other abrasive and successive pigmented .layers deposited. A final layer of -clear lacquer .may then be deposited, to produce a. high gloss4 'l proved dipping process.' In fact, it is one of the features of our invention that. due to the fact of our lacquersl solidifying or setting to a gel im mediateiy upon reaching the critical temperature filsrange of the solution in' question, no appreciable running or accretions of cellulose .ester materials faceot the article. Articles of wood, metal, paper,

leather. and 'various other materials may be lacquered bssour process with great facility.

whatwe-emma; i A Aoi' coating an article which comprises pr'ep'arlnz Aa. lacquer solution by dissolving a. gellable cellulose ester at a temperature above 'its 1lelatiqn temperature in a solvent composed oi alkyln'e dichlcrides of from 2-4 carbon atoms. article with the solution above its solution-temperature, gelling the coating and removina 'the residualsolvent..

, R. FORDYCE.

cansan J. CLARKE. 

